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The calculation of the thermodynamic parameters for the adsorption of methane by zeolite NaX was carried out using the atom-atom approximation, in which the potential energy of the intermolecular interaction of a molecule with the zeolite lattice can be represented as the sum of the interaction energies of the constituent atoms of the molecule with all the oxygen ions and cations of the lattice. The configuration integrals were calculated numerically, different positions and orientations of the molecule in the zeolite cavity being taken into account. To simplify the calculations the large cavity of the zeolite was divided into small cells. The energies of interaction, at the centre of each cell, of C and H atoms with the lattice were calculated by computer. At any point inside a given cell the corresponding contribution to the energy of interaction was assumed to be equal to that for the centre of the cell. For very small (zero) filling of the zeolite cavity the thermodynamic adsorption characteristics (Henry's constants, changes in differential internal energy, isosteric heats of adsorption and heat capacities of the adsorbed substance) were calculated. By making a reasonable choice of the molecular parameters the atom-atom approximation gives thermodynamic quantities which are in satisfactory agreement with corresponding experimental values.

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The hydroxyl groups of silica (aerosils and silica gels) are divided into surface and bulk groups. The surface OH groups are rapidly substituted by OD, making it possible to determine the surface concentration of hydroxyl groups by exchange between the samples and D20 vapour at ordinary temperatures, as well as the total amount of OH groups in the sample by exchange between D2O vapour and the entire quantity of water evolved when the sample is heated to 1200°C. The isotopic composition of the vapour after deuterium exchange was determined by mass spectrometry. Deuterium exchange was also studied by an infra-red spectroscopy. The ratio between the quantities of surface and bulk OH groups depends on the pre-treatment of the sample and on the size of the silica globules. The concentration of OH groups on the surface of maximally hydroxylated samples is practically independent of their history and dispersity. Surface OH groups may be subdivided in turn into free and bound groups. The main role in specific adsorption (when hydrogen and similar bonds are formed with the adsorbate molecules) and in the chemical reaction with trimethylchlorosilane must be credited to the free hydroxyl groups of the silica surface.* the measurement by I. Yu. Babkin.

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